Interpretive Summary: The European corn borer injures corn leaves, stalks, shanks, and ears when it is in the caterpillar stage. Corn hybrids [Bacillus thuringiensis (Bt) corn] that have been genetically modified to contain a bacterial gene, which is toxic to moth caterpillars but nothing else, are used to manage the moth in North America. However, there is concern that this insect pest may become resistant to the toxin, making Bt corn ineffective. To prevent this, a common non-Bt corn hybrid is planted as a refuge within a prescribed distance of Bt corn to allow survival of susceptible moths to mate with resistant moths. Mating often occurs in grassy areas near cornfields where moths aggregate. The appropriate distance between a refuge and Bt corn requires an understanding of moth movement and mating behavior, but much remains unknown. We examined the influences of a male-attractant lure, aggregation-site plant density, and plant species on the distribution of moths among small-grain aggregation sites. Wild moths were collected in aggregation sites in relative abundance, indicating the sites were acceptable. In contrast, newly-emerged, artificially-marked moths that were released weekly were rarely found in the aggregation sites, suggesting that young moths fly away from their field of origin and do not land in the first acceptable aggregation sites encountered. Based on moths recovered, plant species significantly influenced moth distributions among aggregation sites. Understanding adult dispersal behavior is important because it directly affects mating patterns and the rate at which Bt-resistance genes move through a population. These results will help scientists design better refuge plans and better strategies for slowing the spread of resistance once it develops.

Technical Abstract:
Genetically-modified, insecticidal corn hybrids [Bacillus thuringiensis (Bt) corn] are used throughout the U.S. Corn Belt for European corn borer, Ostrinia nubilalis (Hubner), control. To slow development of Bt-corn resistance, the EPA requires provision of nearby refugia. The appropriate distance between a refuge and Bt corn, and development of mitigation-remediation strategies such as mass releases of laboratory-reared, susceptible moths, require an understanding of adult dispersal and mating behavior. However, much remains unknown about such behaviors. Mating often occurs in grass near cornfields where moths aggregate, and we planted small-grain plots in an attempt to retain mass released moths. The objectives of this study were to examine influences of pheromone lure, plant density, and plant species on the distribution of feral and newly-emerged, laboratory-reared O. nubilalis among small-grain aggregation plots. Feral moths were collected in aggregation plots in relative abundance, indicating that plots were acceptable aggregation sites. In contrast, newly-emerged moths that were released weekly as dye marked pupae were rarely found in aggregation plots, with approximately 150 to 1,500-fold fewer moths captured than expected if all the released moths had occupied any of the plots for greater than or equal to 1 d. The majority of newly-emerged adults did not colonize the aggregation plots, suggesting that recently-eclosed adults leave their natal field and do not colonize the first aggregation sites encountered. Plant species significantly influenced moth distributions among aggregation plots. Mass releases of laboratory-reared pupae in the field may not be a viable mitigation tactic because almost all of the newly-emerged moths dispersed beyond 300 m of the release point.